This review highlights recent advances in environmentally benign approaches toward mesoionic oxazolium and thiazolium derivatives, emphasizing solvent-free reactions, microwave-assisted synthesis, ultrasonic irradiation, aqueous and bio-based solvent systems, recyclable catalysts, and one-pot multicomponent strategies. Particular attention is given to atom economy, energy efficiency, reduced waste generation, and the use of non-toxic reagents. Comparative evaluation of conventional and green protocols demonstrates that sustainable methodologies not only minimize environmental impact but often enhance reaction efficiency, selectivity, and scalability. The integration of green chemistry principles into heterocyclic synthesis provides promising pathways for the development of eco-friendly and industrially viable processes for mesoionic compounds. Mesoionic oxazolium and thiazolium derivatives represent a unique class of heterocyclic compounds characterized by delocalized charge distribution and remarkable chemical versatility. These frameworks, closely related to N-heterocyclic carbene precursors, have attracted significant attention due to their applications in catalysis, medicinal chemistry, materials science, and agrochemistry. In recent years, increasing environmental concerns and the principles of green chemistry have driven the development of sustainable synthetic methodologies for constructing these mesoionic systems.